JP7440740B2 - Steel plate for tanks - Google Patents

Description

The present invention relates to a steel plate for tanks.

When energy resources such as liquefied petroleum gas (hereinafter also simply referred to as "LPG") and ammonia gas are transported by sea, they are transported by energy transport ships. From the viewpoint of transportation efficiency, these gases are compressed and liquefied and stored in tanks provided inside the ship.

In order to maintain a liquid state in the tank, the gas is pressurized at high pressure and is at cryogenic temperatures. Therefore, thick steel plates used as materials for tanks are required to have low-temperature toughness that can withstand the above-mentioned temperatures. Furthermore, gases such as ammonia may cause stress corrosion cracking. For this reason, materials for tanks are required to have low yield ratio characteristics (also referred to as "low YR characteristics") in order to suppress the occurrence of stress corrosion cracking.

For example, Patent Document 1 discloses a steel material used for tanks for LPG and ammonia gas transport ships.

Japanese Patent Application Publication No. 2008-25014

By the way, with the increase in energy demand in recent years, the demand for energy transport ships is increasing. For this reason, there is a need for larger transport ships. As transport ships become larger, the amount of steel plates used also increases. Therefore, from the viewpoint of reducing the construction cost of transport ships, there is a need for a steel plate for tanks that has the above-mentioned characteristics but has a further reduced alloy cost.

An object of the present invention is to solve the above problems and provide a steel plate for a tank that has excellent low-temperature toughness, a low yield ratio, and is low in cost.

The present invention has been made to solve the above problems, and its gist is the following steel plate for tanks.

(1) The chemical composition is in mass%,
C: 0.04-0.10%,
Si: 0.05-0.50%,
Mn: 0.9 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Ti: 0.005 to 0.025%,
Cr: 0.10-0.20%,
sol. Al: 0.005-0.090%,
N: 0.001 to 0.010%,
The remainder: Fe and impurities,
At the 1/4 plate thickness position, the metal structure is
In area ratio,
Ferrite phase: 40.0 to 70.0%,
Hard phase: 3.0% or less,
Island martensite: 0.1% or less,
The rest: Bainite,
The average grain size of the ferrite phase is 6.0 to 12.0 μm,
A steel plate for tanks, wherein the standard deviation σ in the grain size distribution of the ferrite phase is 4.0 to 7.0 μm.

(2) The chemical composition is in mass % instead of a part of the Fe,
Nb: 0.015% or less,
Cu: 0.50% or less,
Ni: 0.40% or less,
Mo: 0.20% or less,
The steel plate for a tank according to (1) above, containing one or more types selected from V: 0.06% or less, and B: 0.002% or less.

(3) The chemical composition is in mass % instead of a part of the Fe,
The steel plate for a tank according to (1) or (2) above, containing Ca: 0.005% or less.

(4) The chemical composition is in mass % instead of a part of the Fe,
The steel plate for a tank according to any one of (1) to (3) above, containing Mg: 0.005% or less.

(5) The steel plate for a tank according to any one of (1) to (4) above, which is used for a tank for a carrier ship for LPG and/or liquefied ammonia.

According to the present invention, it is possible to obtain a steel plate for a tank that has excellent low-temperature toughness, a low yield ratio, and is low in cost.

The present inventors conducted various studies in order to obtain a steel plate for a tank that has good low-temperature toughness, a low yield ratio, and is low in cost. As a result, we obtained the following knowledge.

(a) In order to realize a low-cost steel plate, it is effective to reduce the amount of alloying elements used. Conventionally, many alloying elements have been used in steel plates used for tanks of LPG and liquefied ammonia carriers. Among such alloying elements, Cu and Ni are particularly expensive, so it is desirable to reduce the amount of them contained in the steel sheet.

(b) On the other hand, since Cu and Ni are elements that contribute to improving strength, reducing the content of these alloying elements reduces the strength. Therefore, in order to supplement the strength, it is effective to adjust the content of Cr, which is an inexpensive alloying element compared to Cu and Ni.

(c) On the other hand, Cr is an element that has the effect of stabilizing the austenite phase. Therefore, when the Cr content is increased, the austenite phase is stabilized during the manufacturing process, making it difficult to form a homogeneous ferrite phase. If a homogeneous ferrite phase is not formed, it becomes difficult to stably produce a steel plate that has good low-temperature toughness and a low yield ratio.

(d) In order to make the ferrite phase homogeneous, it is desirable to perform preliminary water cooling before water cooling in the cooling step in the manufacturing process. Thereby, the driving force for transforming the austenite phase into the ferrite phase (also referred to as "transformation driving force") can be improved, and the transformation can be promoted. As a result, a steel plate having a homogeneous ferrite phase can be obtained.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be explained in detail.

1. Chemical composition The reasons for limiting each element are as follows. In addition, in the following description, "%" regarding content means "mass %".

C: 0.04-0.10%
C is an extremely effective element for increasing the strength of steel materials. If the content is less than 0.04%, the desired strength cannot be ensured, and the microstructure becomes insufficiently refined, resulting in deterioration of low-temperature toughness. Therefore, the C content is set to 0.04% or more. The C content is preferably 0.045% or more. However, if the content exceeds 0.10%, the yield ratio increases due to an increase in yield strength, impairing stress corrosion cracking resistance. Therefore, the C content is set to 0.10% or less.

Si: 0.05-0.50%
Si is a necessary element as a deoxidizing agent in the deoxidizing process, and is also useful as a ferrite-forming element. In order to obtain a sufficient deoxidizing effect and a sufficient amount of ferrite, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more. However, if Si is contained in excess of 0.50%, the weld heat affected zone (hereinafter also referred to as "HAZ") will be abnormally hardened, leading to a decrease in joint toughness. Furthermore, the ferrite grain size also tends to increase.
Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.40% or less.

Mn: 0.9-2.0%
Mn is an element that improves the hardenability of steel and increases the strength and toughness of steel materials. If the Mn content is less than 0.9%, these effects cannot be obtained. Furthermore, the crystal grain size of the ferrite phase tends to vary, and as a result, the yield ratio increases. Therefore, the Mn content is set to 0.9% or more. However, when the Mn content exceeds 2.0%, the toughness of the HAZ deteriorates during welding. For this reason, the Mn content is preferably 2.0% or less.

P: 0.020% or less P is generally contained as an impurity, and if it is contained in an amount exceeding 0.020%, it will segregate at grain boundaries and become a starting point for sulfide stress corrosion cracking. Therefore, the P content is set to 0.020% or less. It is desirable to reduce P as much as possible.

S: 0.010% or less S is generally contained as an impurity, combines with Mn in steel to form MnS, and deteriorates the low-temperature toughness and ductility of the steel material. Therefore, the S content is set to 0.010% or less. It is desirable to reduce S as much as possible.

Ti: 0.005-0.025%
Ti is an element that combines with N in steel to form TiN and improves the cleanliness of the slab surface and the steel material surface. Furthermore, it has the effect of suppressing coarsening of austenite crystal grains. Therefore, the Ti content is set to 0.005% or more. However, if Ti is contained in an amount exceeding 0.025%, the precipitates may become coarse and the toughness of the base material may deteriorate. Therefore, the Ti content is set to 0.025% or less.

Cr:0.10~0.20%
Cr is an element that stabilizes austenite when contained in an appropriate amount and is effective in improving cryogenic toughness and base material strength. When the Cr content is low, the crystal grain size of the ferrite phase tends to vary, and as a result, the yield ratio also increases. Therefore, the Cr content is set to 0.10% or more. However, when Cr is contained in an amount exceeding 0.20%, the strength of the steel material becomes too high. Therefore, the Cr content is set to 0.20% or less.

sol. Al: 0.005-0.090%
Al acts as a deoxidizing agent and is most commonly used in the process of deoxidizing molten steel sheets. Furthermore, by fixing solid solution N in the steel to form AlN, it has the effect of suppressing coarsening of crystal grains. sol. If the content as Al (“acid-soluble Al”) is less than 0.005%, this effect cannot be obtained. For this reason, sol. Al content shall be 0.005% or more. However, sol. If the Al content exceeds 0.090%, it will be mixed into the weld metal during welding and deteriorate the toughness of the weld metal, so the Al content is set to 0.090% or less.

N: 0.001-0.010%
N combines with Ti to form nitrides and has the effect of suppressing coarsening of crystal grains in the austenite phase. Therefore, the N content is set to 0.001% or more. The N content is preferably 0.002% or less. However, if the N content exceeds 0.010%, the nitrides become coarse and the toughness decreases. Therefore, the N content is set to 0.010% or less. The N content shall be 0.008% or less.

Nb: 0.015% or less Nb is an effective element for expanding the unrecrystallized austenite region, contributes to grain refinement, and improves strength and toughness. Therefore, it may be included if necessary. However, if the Nb content exceeds 0.015%, the crystal grains of the ferrite phase become significantly finer, making it impossible to achieve a low YR. Therefore, the Nb content is set to 0.015% or less. The Nb content is preferably less than 0.005%. On the other hand, in order to obtain the above effects, it is preferable that Nb be 0.001% or more, more preferably 0.002% or more.

Cu: 0.50% or less Cu is an element that increases the strength and corrosion resistance of steel materials, so it may be included if necessary. However, if the Cu content is excessive, hot cracking is likely to occur. Therefore, the Cu content is set to 0.50% or less. The Cu content is preferably 0.40% or less. On the other hand, in order to obtain the above effects, the Cu content is preferably 0.05% or more.

Ni: 0.40% or less Ni is a typical austenite stabilizing element and is an effective element for improving cryogenic toughness and base material strength, so it may be included as necessary. However, excessive Ni content increases the cost of the steel material and is economically disadvantageous. Therefore, the Ni content is set to 0.40% or less. On the other hand, in order to stably obtain the above effects, the Ni content is preferably 0.05% or more.

Mo: 0.20% or less Mo is an element that increases the strength of steel materials, and also contributes to grain refinement of the ferrite phase, so it may be included as necessary. However, when the Mo content becomes excessive, the crystal grains of the ferrite phase become significantly finer, and the yield strength increases and the yield ratio increases. It also causes a significant decrease in weldability. Therefore, the Mo content is set to 0.20% or less. The Mo content is preferably 0.15% or less. On the other hand, in order to obtain the above effects, the Mo content is preferably 0.005% or more.

V: 0.06% or less V forms carbonitrides and has the effect of precipitation strengthening steel materials, so it may be included as necessary. However, when the V content is excessive, not only does the effect become saturated, but also the amount of fine precipitates produced increases, resulting in a decrease in toughness. Therefore, the V content is set to 0.06% or less. The V content is preferably 0.05% or less. On the other hand, in order to stably obtain the above effects, the V content is preferably 0.005% or more.

B: 0.002% or less B is an element that improves the hardenability of steel materials in small amounts, and may be included as necessary. However, when the B content is excessive, precipitates are generated and the toughness is reduced. Therefore, the B content is set to 0.002% or less. The B content is preferably 0.0015% or less. On the other hand, in order to stably obtain the above effects, the B content is preferably 0.0002% or more.

Ca: 0.005% or less Ca combines with S in steel to form a Ca-Mn-S compound. As a result, Ca is extremely effective in inhibiting the evolution of Mn--S compounds and reducing the anisotropy of the mechanical properties of steel. Therefore, it may be included if necessary. However, since the effect is saturated even if Ca is contained in an amount exceeding 0.005%, the Ca content is made 0.005% or less. The Ca content is preferably 0.004% or less. On the other hand, in order to stably obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.0020% or more.

Mg: 0.005% or less Mg combines with O or S in the steel and suppresses the growth of crystal grains in the austenite phase of the HAZ. It may be included as necessary, such as when toughness of the HAZ is required. However, when the Mg content exceeds 0.005%, the above effects are saturated. Therefore, the Ca content is set to 0.005% or less. The Mg content is preferably 0.004% or less. On the other hand, in order to stably obtain the above effects, the Mg content is preferably 0.001% or more, more preferably 0.002% or more.

In the chemical composition of the present invention, the remainder is Fe and impurities. Here, "impurities" are components that are mixed in during the industrial production of steel due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are allowed within the range that does not adversely affect the present invention. means something that

2. Metal structure In the steel plate of the present invention, the metal structure at the 1/4 position of the plate thickness, which is considered to be the average structure, in the entire thickness of the steel plate, specifically, the area ratio of each phase, the average grain size of the ferrite phase, stipulates.
2-1. Area ratio of each phase The steel plate according to the present invention has a metal structure in area ratio of ferrite phase: 40.0 to 70.0%, hard phase: 3.0% or less, island Martensite: 0.1% or less, remainder: bainite.

If the area ratio of the ferrite phase is less than 40.0%, the amount of bainite or hard phase will increase relatively. As a result, the yield stress increases and the stress corrosion cracking resistance deteriorates. However, if the area ratio of the ferrite phase exceeds 70.0%, the required tensile strength cannot be obtained and the material cannot be applied to large tanks or the like. Therefore, the area ratio of the ferrite phase is set to 40.0 to 70.0%.

Note that the area ratio of the ferrite phase refers to the area ratio of a region where the GAM (Grain Average Misorientation) value obtained by the EBSD method is 0.5 or less.

The area ratio of the unavoidably generated hard phase is 3.0% or less. Here, the hard phase refers to a martensitic phase (not including island martensite) or pearlite, and it is sufficient if the total area ratio of these phases is 3.0% or less. This is because if the hard phase is contained in an amount exceeding 3.0%, the toughness will be significantly deteriorated. It is preferable not to generate a hard phase as much as possible. Incidentally, island martensite is generally defined as a hard phase, but here, as shown below, since it has a large effect on toughness, in the present invention, the hard phase does not include island martensite (more precisely, it is defined as Martensite does not include island martensite) and is defined separately.

Furthermore, when microstructure is observed, island-shaped martensite may be observed in bainite. If a large amount of this island martensite is generated, it will have an adverse effect on toughness, and even when welded, it will cause a decrease in HAZ toughness. Therefore, the area ratio of the island martensite is 0.1% or less. The remainder other than the ferrite phase, hard phase, and island martensite is bainite.

Note that the area ratio of each phase is measured according to the following procedure. Specifically, for the area ratio of the ferrite phase, a part of the test piece was cut out from 1/4 of the thickness of the test piece, the L cross section was mirror polished, and then final polishing was performed for 30 minutes with colloidal silica. , an EBSD (electron beam backscatter diffraction) method was used. The obtained data is analyzed using OIM-Analysis software (hereinafter referred to as "OIM software") manufactured by TSL Solutions Inc. using a GAM value representing the average value of crystal orientation differences within crystal grains. At this time, a GAM value of 0.5 or less is defined as a ferrite phase, and the area ratio of the ferrite phase is calculated. Note that the photographing field of view was set to five fields, and the obtained area ratios were averaged to determine the area ratio of the ferrite phase.

Regarding the area ratio of the hard phase, a part of the test piece was cut out from 1/4 of the thickness of the test piece, the L cross section was polished to a mirror finish, and then etched with a 3% nital solution, and then examined using an optical microscope at 500x magnification. The microstructure is photographed, and the area ratio of the martensite phase and pearlite phase is determined by image processing. Note that the photographic field of view was set to five fields, and the obtained area ratios were averaged to determine the area ratios of the martensite phase and pearlite phase.

Regarding the area ratio of island-like martensite, the L cross section is mirror-polished and repeller-corroded, the microstructure is photographed with an optical microscope at a magnification of 500 times, and the area ratio is determined by image analysis. Note that the photographic field of view was set to five fields, and the obtained area ratios were averaged to determine the area ratio of island-like martensite.

2-2. Average grain size of ferrite phase In the steel sheet according to the present invention, the average grain size of the ferrite phase at the 1/4 position of the plate thickness is 6.0 to 12.0 μm. Generally, in order to improve the strength and toughness of a steel sheet, it is effective to reduce the grain size of the ferrite phase. If it is less than 0 μm, yield stress increases and stress corrosion cracking resistance deteriorates. Therefore, the average crystal grain size of the ferrite phase is set to 6.0 μm or more. The average crystal grain size of the ferrite phase is preferably 7.0 μm or more. However, when the average grain size of the ferrite phase exceeds 12.0 μm, the toughness deteriorates significantly. Therefore, the average crystal grain size of the ferrite phase is set to 12.0 μm or less. The average grain size of the ferrite phase is preferably 10.0 μm or less.

2-3. Standard Deviation in Crystal Grain Size of Ferrite Phase In the steel sheet according to the present invention, the standard deviation σ in the crystal grain size distribution of the ferrite phase at the 1/4 position of the plate thickness is 4.0 to 7.0 μm. If the standard deviation is less than 4.0 μm, more alloying elements such as arbitrary elements are required, resulting in an increase in alloy cost. Therefore, the standard deviation σ is set to 4.0 μm or more. However, when the standard deviation σ exceeds 7.0 μm, variations in toughness become large and the characteristic value of toughness may decrease. Therefore, the standard deviation σ is set to 7.0 μm or less.

Note that the crystal grain size described in 2-2 and 2-3 above refers to the equivalent circle diameter in a region surrounded by large-angle grain boundaries of 15 degrees or more by the EBSD method. When calculating the area ratio of the ferrite phase, the average grain size of the ferrite phase is determined by the OIM software manufactured by TSL Solutions. can be calculated as the average value of the equivalent circle diameters measured in the observation field. Further, the standard deviation in the crystal grain size distribution of the ferrite phase can be calculated using OIM software in the range of 3σ for the measured circle equivalent diameter.

3. Plate Thickness The plate thickness of the steel plate according to the present invention is not particularly limited. The plate thickness normally required for tanks is 10 mm or more, and is 50 mm at most. The steel plate according to the present invention preferably has a thickness of 10 to 50 mm, more preferably 15 to 40 mm.

4. Application The steel plate according to the present invention is used for tanks, and is particularly suitably used for tanks for LPG and/or liquefied ammonia carriers.

5. Targeted Properties The steel plate according to the present invention is evaluated to have a low yield ratio when the yield ratio (YR) is 80% or less. Furthermore, when the brittle-ductile transition temperature is -60°C or lower, it is evaluated as having excellent low-temperature toughness.

6. Manufacturing Method As mentioned above, it is important to lower the yield ratio in order to improve stress corrosion cracking properties. For this reason, it is necessary to ensure a certain amount of ferrite phase in the metal structure. As a method for this purpose, a ferrite phase may be precipitated by cooling for a certain period of time after completion of rolling, and then water cooling may be performed.

However, in such a conventional method, it is necessary to temporarily stop the steel plate on the production line. Therefore, there is a problem that manufacturing efficiency is reduced. Further, due to the scale that is non-uniformly generated during cooling, the cooling rate during water cooling locally changes, resulting in a problem that the mechanical properties of the steel sheet vary widely.

Thus, from the viewpoint of production efficiency, it is not preferable to temporarily stop the steel plate on the production line. On the other hand, if the steel plate is not stopped on the production line, it will not be possible to allow it to cool sufficiently and will have to perform accelerated cooling such as water cooling, making it difficult to secure a sufficient amount of ferrite phase.

Based on the above points, as a result of repeated studies by the present inventors, it is effective to perform preliminary cooling (primary cooling) such as water cooling before accelerated cooling (secondary cooling) such as water cooling, which will be described later. As a result, a steel plate with a low yield ratio can be manufactured without stopping the steel plate on the production line.

At this time, by performing primary cooling immediately after finish rolling, the strain in the structure of the austenite phase introduced by controlled rolling increases the driving force for transformation to the ferrite phase, and the primary cooling and secondary cooling increase. During cooling, ferrite transformation can be efficiently promoted.

There are no particular limitations on the method for manufacturing the steel sheet according to the present invention as long as it is possible to manufacture the steel sheet having the chemical composition and metal structure described above. I can do it.

in particular,
(a) heating a slab having a thickness of 200 mm or more and having the above chemical composition to a temperature range of 1050 to 1200 °C;
(b) After rolling with a cumulative reduction rate of 30% or more in a slab surface temperature range of 750 to 900°C, rolling is completed under conditions such that the steel plate surface temperature becomes 750°C or higher after rolling. and,
(c) a first cooling step in which water cooling is started when the steel plate surface temperature is 750°C or higher, the average cooling rate is 10°C/s or higher, and water cooling is stopped in a temperature range of 600 to 750°C;
(d) cooling the steel plate in air for 10 to 60 seconds from the end of the first cooling to the start of the step (e);
(e) a second cooling step in which water cooling is started when the steel plate surface temperature is in a temperature range of 600 to 700°C, the average cooling rate is 15°C/s or more, and water cooling is stopped in a temperature range of 550°C or less;
(f) Reheating to a temperature of 450°C or higher and Ac ₁ point or lower, soaking for 30 minutes or more, and then cooling;
has. Each step will be explained below.

6-1. Hot Rolling Process First, it is preferable to prepare a slab of 200 mm or more having the above-mentioned chemical composition. Preferably, the slab is heated in a heating furnace at a temperature in the range of 1050 to 1200°C. By setting the heating temperature during heating to 1050°C or higher, it is possible to suppress the crystal grains of the austenite phase from being excessively refined, and it is easier to optimize the crystal grains of the ferrite phase and the strength of the steel sheet. can. For this reason, the heating temperature of the slab is preferably 1050°C or higher.

On the other hand, when the heating temperature of the slab exceeds 1200° C., the crystal grains of the austenite phase become coarse. In this case, the low-temperature toughness of the steel plate decreases. For this reason, the heating temperature of the slab is preferably 1200°C or less.

The slab is taken out of the heating furnace and hot rolled. At this time, it is preferable that the cumulative reduction rate is 30% or more when the surface temperature of the slab during rolling is 900° C. or less. By setting the cumulative reduction rate to 30% or more when the surface temperature of the slab is 900° C. or lower, coarsening of crystal grains can be suppressed and better low-temperature toughness can be ensured. Therefore, it is preferable to carry out rolling at a slab surface temperature of 900° C. or lower and a cumulative reduction rate of 30% or higher. Moreover, it is more preferable to carry out rolling with a cumulative reduction ratio of 35% or more.

Furthermore, by setting the surface temperature of the slab during rolling to 750° C. or higher, the formation of pro-eutectoid ferrite can be suppressed and the driving force for transformation can be obtained more reliably. For this reason, the surface temperature of the slab during rolling is preferably 750°C or higher. Then, it is preferable to complete the rolling under the condition that the steel plate surface temperature becomes 750° C. or higher.

In addition, the cumulative rolling reduction rate mentioned above is defined by the following formula (1).
Cumulative rolling reduction rate = (Thickness before rolling - Thickness after rolling) / (Thickness before rolling) x 100 (1)

6-2. First cooling process After the above rolling is completed, water cooling is started when the steel plate surface temperature is 750°C or higher, the average cooling rate is 10°C/s or higher, and water cooling is stopped in the temperature range of 600 to 750°C (" (Also referred to as "first cooling step"). This is achieved by water cooling from a temperature of 750°C or higher, where the ferrite phase is generated, while suppressing the amount of coarse ferrite phase generated, increasing the driving force for the generation of the ferrite phase, and allowing it to be generated during the cooling time described below. This is to reduce the crystal grain size of the ferrite phase.

Moreover, in the first cooling step, by setting the average cooling rate to 10° C./s or more, generation of a coarse ferrite phase can be appropriately suppressed during cooling. For this reason, it is preferable that the average cooling rate in the first cooling step is 10° C./s or more.

6-3. Air Cooling Step It is preferable that the steel plate be air cooled for 10 to 60 seconds from the end of the first cooling to the start of the second cooling. By setting the time during air cooling (hereinafter also referred to as "air cooling time") to 10 seconds or more, it becomes easy to secure the amount of ferrite necessary to obtain a steel plate with a low yield ratio. On the other hand, by setting the air cooling time to 60 seconds or less, coarsening of the crystal grains of the ferrite phase can be suppressed, and low-temperature toughness can be appropriately ensured. Furthermore, if the air cooling time exceeds 60 seconds, there is a possibility that the second cooling cannot be started from the optimum temperature range. Therefore, the air cooling time is preferably 60 seconds, more preferably 30 seconds or less.

6-4. Second cooling process Next, water cooling is started when the steel plate surface temperature is in the temperature range of 600 to 700°C, the cooling rate is set to 15°C/s or more, and water cooling is stopped when the steel plate surface temperature is in the range of 550°C or less ("second cooling") Also called.). By setting the water cooling start temperature in the second cooling to 700° C. or lower even if reheating occurs, ferrite transformation can proceed and the amount of ferrite required for a low yield ratio can be obtained. For this reason, it is preferable that the water cooling start temperature in the second cooling is 700° C. or lower.

On the other hand, by setting the water cooling start temperature in the second cooling to 600° C. or higher, ferrite transformation can appropriately proceed. Therefore, the amount of ferrite does not become too large, or the crystal grain size of the ferrite phase does not become too fine. For this reason, it is preferable that the water cooling start temperature in the second cooling is 600° C. or higher.

In addition, in the second cooling step, by setting the average cooling rate to 15°C/s or more, when untransformed austenite undergoes phase transformation to bainite, an upper bainite structure is formed in which cementite precipitates between the laths of bainite. can be suppressed, and deterioration of low-temperature toughness can be prevented. For this reason, it is preferable that the average cooling rate in the second cooling step is 15° C./s or more.

Further, by setting the temperature at which water cooling is stopped (hereinafter also referred to as "water cooling stop temperature") to be 550° C. or lower, a bainite structure with high strength necessary for low YR can be obtained. For this reason, the water cooling stop temperature is preferably 550°C or less. On the other hand, by setting the water cooling stop temperature to 400° C. or higher, martensitic transformation that causes hardness can be suppressed, and a decrease in toughness can be suppressed. Therefore, if it is desired to achieve both toughness and low yield ratio, the water cooling stop temperature is preferably 400° C. or higher.

6-5. Tempering Process Subsequently, it is preferable to perform a tempering process. That is, it is preferable to reheat to a temperature of 450° C. or more and Ac ₁ point or less, hold soaking (tempering) for 30 minutes or more, and then cool. By reheating and holding the steel at a uniform temperature, residual stress in the steel plate can be removed, and the island-like martensite observed in bainite can be decomposed and the negative effect on toughness reduction can be minimized. . After the above-mentioned soaking, cooling is performed. Cooling here may be air cooling or water cooling.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

Steel having the chemical composition shown in Table 1 was melted and a slab was produced using a continuous casting machine. The thickness of the produced slab was 300 mm. The obtained slab was hot rolled under the conditions shown in Table 2, and then subjected to first cooling and second cooling, and test No. Steel No. 31 was removed and tempered to produce steel plates having the thickness shown in Table 3. Note that the Ac ₁ point was 700°C or higher for all steel plates.

(Tissue observation and image analysis)
Regarding the metallographic structure, a part of the test piece was cut out from 1/4 of the thickness of the test piece, the L cross section was mirror-polished, and the microstructure was photographed at 500x magnification using an optical microscope at a magnification of 500. The area ratio of island martensite was determined. For image analysis, Nippon Steel & Sumikin Technology Co., Ltd.'s image processing package software ``Particle Analysis'' was used. In the image analysis, five photographic fields were used, and the obtained area ratios were averaged to determine the area ratio of island-like martensite.

On the other hand, regarding the area ratio of the ferrite phase, a part of the test piece was cut out from 1/4 of the thickness of the test piece, the L cross section was polished to a mirror finish, and after finishing polishing with colloidal silica for 30 minutes, EBSD ( The electron backscatter diffraction method was used. The obtained data was analyzed using OIM software manufactured by TSL Solutions using the GAM value representing the average value of the crystal orientation difference within the crystal grains, thereby attempting to separate the ferrite phase and bainite. At this time, a GAM value of 0.5 or less was defined as a ferrite phase, and the area ratio of the ferrite phase was calculated.

The average crystal grain size of the ferrite phase is determined by calculating the equivalent circle diameter in a region surrounded by grain boundaries with a large angle of 15 degrees or more using OIM software manufactured by TSL Solutions when calculating the area ratio of the ferrite phase mentioned above. , was calculated as the average value of the equivalent circle diameters measured in the observation field. Further, the standard deviation in the crystal grain size distribution of the ferrite phase was calculated using OIM software in the range of 3σ for the measured circle equivalent diameter.

In addition, the area ratio of the hard phase can be determined by cutting out a part of the test piece from the 1/4 position of the test piece, mirror-polishing the L cross section, corroding it with a 3% nital solution, and using an optical microscope at 500x magnification. The microstructure was observed. However, in all test pieces, the hard phase was 3.0 area % or less.

(Tensile test)
A round bar tensile test piece having a parallel portion length of 8.5 mm and a gauge length of 42.5 mm was prepared from each of the obtained steel plates. At this time, the test piece was cut out so that the length direction of the round bar tensile test piece was perpendicular to the rolling direction (plate width direction). A tensile test was conducted at room temperature and atmospheric pressure using a round bar tensile test piece to determine yield strength YS (N/mm2), tensile strength TS (N/mm2), and yield ratio YR (=YS/TS×100, The unit is %).

(Brittle-ductile transition temperature)
Furthermore, low-temperature toughness was evaluated by Charpy impact test. The Charpy impact test was conducted three times each at five different temperatures including -60°C using a V-notch test piece specified in JIS Z 2242:2005, and the temperature at which the brittle fracture ratio was 50% was determined as brittle-ductile. transition temperature.

Here, when the yield ratio (YR) is 80% or less, it is evaluated as a low yield ratio. Furthermore, when the brittle-ductile transition temperature is -60°C or lower, it is evaluated as having excellent low-temperature toughness. The results are summarized in Table 3 below.

Test No. that satisfies the provisions of the present invention. Steel plates Nos. 1 to 15 had a low yield ratio of 80% or less, a brittle-ductile transition temperature of −60° C. or less, and good low-temperature toughness properties.

On the other hand, test No. that does not satisfy the provisions of the present invention. For steel plates No. 16 to 31, at least one of the yield ratio and the brittle-ductile transition temperature did not satisfy the target characteristic value.

Test No. Steel plate No. 16 had a high fracture surface transition temperature due to its high N content, and could not obtain sufficient low-temperature toughness. Test No. Steel plate No. 17 had a high C content, so the area ratio of the ferrite phase was small and the yield ratio was large. Test No. Steel plate No. 18 had a low Cr content, so the standard deviation in the grain size distribution of the ferrite phase was large and the area ratio of the ferrite phase was also small. This also increased the yield ratio.

Test No. Steel plate No. 19 had a high Si content and a high Ti content, so the average grain size of the ferrite phase was also large. As a result, the yield ratio increased. Test No. Steel plate No. 20 had a low Mn content, so the standard deviation in the grain size distribution of the ferrite phase was large, the yield ratio was large, and the fracture surface transition temperature was high. Test No. Steel plate No. 21 had a low cumulative reduction rate of 10% in the range of 750 to 900°C, so the average grain size of the ferrite phase increased and the standard deviation in the grain size distribution also increased. As a result, the fracture surface transition temperature became high, and sufficient low-temperature toughness could not be obtained.

Test No. In steel plate No. 22, the rolling completion temperature was low and the cooling start temperature in the first cooling was also low, so the average grain size of the ferrite phase was large, the standard deviation in the grain size distribution was also large, and the ferrite phase area ratio was also low. has also grown larger. As a result, the yield ratio was high, the fracture surface transition temperature was high, and sufficient low-temperature toughness could not be obtained.

Test No. In steel plate No. 23, since the cooling rate of the first cooling was low, the average crystal grain size of the ferrite phase was large, the standard deviation of the crystal grain size distribution was also large, and the ferrite phase area ratio was also large. As a result, the yield ratio was high, the fracture surface transition temperature was high, and sufficient low-temperature toughness could not be obtained. Test No. In steel plate No. 24, the water cooling stop temperature of the first cooling was low, and as a result, the cooling start temperature of the second cooling was also low, so the standard deviation in the grain size distribution of the ferrite phase was large, and the ferrite phase area ratio was also small. became. This also increased the yield ratio.

Test No. Steel plate No. 25 had a large ferrite phase area ratio because the air cooling time after the first cooling was long, and the standard deviation in the crystal grain size distribution of the ferrite phase was small, so the yield ratio was large. Test No. In steel plate No. 26, since the cooling start temperature of the second cooling was also low, the average crystal grain size of the ferrite phase was large, and the ferrite phase area ratio was also large. As a result, the yield ratio was high, the fracture surface transition temperature was high, and sufficient low-temperature toughness could not be obtained. Test No. In steel plate No. 27, since the cooling rate of the second cooling was low, the average crystal grain size of the ferrite phase became large, and the yield ratio became large.

Test No. In steel plate No. 28, since the slab heating temperature was high, the average crystal grain size of the ferrite phase became large, and the standard deviation of the crystal grain size distribution also became large. As a result, the fracture surface transition temperature became high, and sufficient low-temperature toughness could not be obtained. Test No. In steel plate No. 29, since the cooling start temperature of the first cooling was low, the standard deviation in the crystal grain size distribution of the ferrite phase was large, and the ferrite phase area ratio was also large. As a result, the yield ratio was large, the fracture surface transition temperature became high, and sufficient low-temperature toughness could not be obtained.

Test No. Steel plate No. 30 had a short air-cooling time after the first cooling, so the ferrite phase area ratio became small and the yield ratio became large. Test No. Steel plate No. 31 was not tempered, so island-like martensite existed in the bainite, the fracture surface transition temperature became high, and sufficient low-temperature toughness could not be obtained.

Claims (5)

The chemical composition is in mass%,
C: 0.04-0.10%,
Si: 0.05-0.50%,
Mn: 0.9 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Ti: 0.005 to 0.025%,
Cr: 0.10-0.20%,
sol. Al: 0.005-0.090%,
N: 0.001 to 0.010%,
The remainder: Fe and impurities,
At the 1/4 plate thickness position, the metal structure is
In area ratio,
Ferrite phase: 40.0 to 70.0%,
Hard phase: 3.0% or less,
Island martensite: 0.1% or less,
The rest: Bainite,
The average grain size of the ferrite phase is 6.0 to 12.0 μm,
A steel plate for tanks, wherein the standard deviation σ in the grain size distribution of the ferrite phase is 4.0 to 7.0 μm. The chemical composition is replaced by a part of the Fe in mass %,
Nb: 0.015% or less,
Cu: 0.50% or less,
Ni: 0.40% or less,
Mo: 0.20% or less,
The steel plate for a tank according to claim 1, containing one or more types selected from V: 0.06% or less, and B: 0.002% or less. The chemical composition is replaced by a part of the Fe in mass %,
The steel plate for a tank according to claim 1 or 2, containing Ca: 0.005% or less. The chemical composition is replaced by a part of the Fe in mass %,
The steel plate for a tank according to any one of claims 1 to 3, containing Mg: 0.005% or less. The steel plate for a tank according to any one of claims 1 to 4, which is used for a tank for a carrier ship for LPG and/or liquefied ammonia.